GB2083944A - CO2 Laser with Catalyst - Google Patents

Abstract

A CO2(-N2-He) laser has the lasing medium in contact with a catalyst mounted upon a metallic substrate, the catalyst catalysing the reaction 2CO+O2=2CO2 to alleviate degradation of laser performance due to breakdown of CO2 by the electric discharge in a sealed system The metallic substrate is preferably an aluminium-containing ferritic steel having a surface coating of alumina in which the catalyst (for example platinum or palladium) is impregnated. A current may be driven through the catalyst to heat it to 200 DEG C. The metallic substrate provides a rugged catalyst system which is not brittle and not sensitive to fracture when subjected to the temperature changes involved inside a laser which may be frequently switched on and off. By impregnating the catalyst in a porous alumina surface on ferritic steel, a large catalytic surface area is available.

Description

SPECIFICATION Improvements in or Relating to Carbon Dioxide Lasers The invention concerns improvements in or relating to COz lasers and particularly to the use of a catalyst in a CO2 laser for improving its performance.

A typical CO2 laser includes a resonant cavity within a gas-tight envelope containing a mixture of CO2, N2 and He which serves as the lasing medium. An energy source is provided for pumping the CO2 to an excited molecular state in order to induce lasing. Typically the energy source consists of a uniform electric discharge across the lasing medium between a principal anode discharge electrode and a principal cathode discharge electrode.

In a 'TE' (Transversely Excited') laser the discharge occurs transverse of the laser axis, this mode of operation often being used in high pressure CO2 lasers.

A disadvantageous phenomenon in CO2 lasers is the breakdown of CO2 into CO and 02 under the influence of the electric discharge. Unlike an unsealed flowing gas system in which the gaseous products are continuously replaced, in a sealed laser the breakdown products remain in the lasing medium and result in degradation of laser performance, for example by causing the uniform electric discharge to break down into localised arcs, thus resulting in a iowering of output power and laser life. The problem becomes more severe as the CO2 pressure in the laser increases.

A known method of alleviating the problem of arcing involves the use of "trigger wires" in a 'TEA' ('Transversely Excited Atmospheric' Pressure) laser, the wires serving to cause subsidiary discharges to the principal anode with resulting UV irradiation of the principal cathode.

This induces diffuse photoemission of electrons from the principal cathode, thus encouraging a uniform discharge.

A second known method of alleviating the problem of breakdown of the uniform principal discharge which is often used in high pressure CO2 lasers involves irradiation of the lasing medium in the discharge volume by a UV source prior to firing the discharge. The UV causes release of electrons by photoionisation of the medium (ie "pre-ionisation") thus promoting a uniform discharge between the principal discharge electrodes.

A third known method of alleviating the problem in sealed CO2 lasers in the inclusion of a heated platinum wire in the lasing medium, the platinum catalysing the recombination of the CC2 dissociation products, CO and 02. A disadvantage of this method of catalysis is that the platinum wire must be heated to an operating temperature of approximately 10000C to be effective. This therefore entails a high power consumption with a consequent high power supply weight and also causes deterioration in the quality of the output beam due to heating and expansion of the resonant cavity. Additionally, the life of the platinum wire is limited due to its fragility.

The purpose of the invention is to provide a CO2 laser in which the problem of arcing due to the dissociation of CO2 into CO and 02 is alleviated by the inclusion of a catalyst system containing a catalyst which does not require a high operating temperature and which is not fragile.

According to the present invention, there is provided a CO2 laser having a lasing medium which includes CO2 wherein the lasing medium is in contact with a catalyst system comprising a catalyst deposited on a metallic substrate, the catalyst being such as to promote combinations of CO and 02 to CO2.

The catalyst is preferably platinum or palladium or both and the substrate may be aluminiumcontaining ferritic steel such that the catalyst is incorporated in an aluminium oxide coating on the surface of the steel. The steel may additionally contain yttrium and chromium and be of the "Fecralloy" (Registered Trade Mark of the United Kingdom Atomic Energy Authority) class of steels.

The catalyst system may take the form of a strip of substrate having the catalyst deposited thereon connected to a current supply so that an electronic current is passed through the strip in order to heat it to an optimum operating temperature.

The invention will now be described by way of example only with reference to the accompanying diagrams of which Figure 1 shows diagrammatically in partial cross-section a plan view of a sealed, pulsed, UV pre-ionisation, TEA CO2 laser Figure 2 shows a side elevation in crosssection of the laser in Figure 1.

Figure 3 is a circuit diagram for the laser of Figure 1 and Figure 2.

Figure 4 is a cross-section of a 'CW' ('Continuous Wave') waveguide CO2 laser.

In Figure 1 a sealed, pulsed UV pre-ionisation TEA CO2 laser having an axis, 5, includes a sealed envelope, 1, of glass and expansion-matched Ni Fe-Co alloy enclosing a lasing medium, 2, consisting of a mixture of 40% CO2, 20% N2 and 40% He (percentages by volume) at 1 atm pressure.

A fully reflecting plane mirror of gold-plated copper, 3, and an 85% reflecting plane mirror of multilayer dielectric-coated germanium, 4, both perpendicular to the laser's axis, 5, define two ends of the laser's resonant cavity.

Pressed nickel Rogowski-profiled cathode, 6, and anode, 7, mounted in alumina insulation, 8 and 9, and separated by alumina spacers, 10 and 11, constitute principal discharge electrodes which provide means of inducing a pulsed electric discharge transverse of the laser axis.

Connections to the electrodes are made via cylinder copper electrical connection tubes, 1 2 and 13.

UV pre-ionisation radiation results from sequential sparking across two series of six spark gaps, one series down a first side of the inside of the envelope, 1, and the other series inside the envelope on the side opposite the first side. The spark gaps, one of which is labelled 14 in Figure 2, are 2 mm wide and lie between pointed ends of tungsten 'T' pins, two of which are labelled 1 5a and 15b in Figure 2. The 'T' pins are connected to earth via 4pF capacitors, two of which are labelled 16 and 1 7 in Figure 2 and Figure 3.

Energy for the spark discharge is provided by a 30 kV HT supply labelled 18 in Figure 3. This charges 900 pF capacitors 1 9 and 20 which then discharge to generate sparks in sequence between the tungsten 'T' pins when spark gap 21 is triggered. The sparks thus generated between the 'T' pins result in UV radiation which preionises the lasing medium.

Following the triggering of spark gap 21 and the pre-ionisation of the lasing medium, discharge between the principal discharge electrodes, 6 and 7, is induced after a delay time by triggering spark gap 22 which allows a 10 nF capacitor, 23, which is charged by 30 kV HT supply, 24, to discharge to earth between electrodes 6 and 7. The preionisation of the medium before the triggering of spark gap, 22, promotes uniform discharge between the principal discharge electrodes.

A variable delay, 25, connected between the two spark gaps, 21 and 22, governs the delay time between the triggering of the two.

On inducing the uniform discharge between the principal discharge electrodes, the CO2 is pumped to an excited molecular state, thus resulting in lasing and causing a pulse of laser radiation to be emitted from mirror 4.

The principal discharge also has the effect of causing dissociation of some of the CO2 to CO and 02. In order to counteract this, a catalyst system in the form of a strip, 26, (in Figure 1) of Fecralloy having a platinum catalyst deposited thereon lies inside the envelope, 1, in contact with the lasing medium 2. Electrical connections to the strip, 26, are made through gas-tight joints, 27 and 28, in the envelope to a current supply. In use, a current is driven through the strip in order to heat it to a temperature of 2000C, at which temperature the platinum catalyst is active in promoting recombination of CO and 02 to CO2.

Use of a platinum catalyst suitably deposited on a Fecralloy substrate provides a catalytic system surface of a porous alumina film containing numerous small particles of platinum.

Consequently, a large active area of catalyst is available within a small area of catalyst system.

The catalyst system can therefore operate at a low temperature and be of small size, thus enabling it to consume a minimum of power and be rugged.

In Figure 4 a CW waveguide CO2 laser includes an outer envelope, 30, and an inner invar envelope,31, which a space, 41, lying between outer and inner envelopes.

An alumina waveguide, 32, having a bore, 33, lies inside the inner envelope, the bore connecting with the space, 41.

A gaseous lasing medium which includes CO2 fills the space, 41, the bore, 33, and a reservoir, 34, which replenishes the medium in the space and bore.

Inside the reservoir, 34, and in contact with the lasing medium is a catalyst system, 42, consisting of a strip of Fecralloy substrate having a palladium catalyst deposited thereon.

Electrically conducting connections pass from the catalyst system through gas-tight joints, 43 and 44, to a current supply outside the laser. In operation, a current is passed through the catalyst system so as to heat it to a temperature of 2000 0.

A fully reflecting mirror, 35, lies at one end of the waveguide, an 85% reflecting mirror, 36, lying at the opposite end, whence an output laser beam is obtained.

Energy is supplied to the medium to induce lasing by two sets of principal discharge electrodes, a first set consisting of a principal cathode, 37, which is at earth potential and a principal anode, 38, which is at a HT supply voltage and a second set consisting of a principal cathode, 39, at earth potential and a principal anode, 40, connected to the same HT voltage as anode 38.

In operation a uniform electric discharge is induced between the two sets of principal discharge electrodes across the lasing medium present in the bore of the waveguide between the electrodes. The discharge causes the CO2 to be pumped to an excited molecular state and consequently causes lasing, an output laser beam exiting from the waveguide via mirror 36.

Heat generated in the waveguide during operation is dissipated by a coolant, 42 contained between the waveguide and the inner envelope.

The electric discharge in the bore of the waveguide results in the dissocication of some of the CO2 contained therein into CO and 02. This is counteracted by the catalyst system, 42, which promotes the recombination of CO and 02 to CO2.

The invention is not restricted to the use of platinum or palladium catalyst on Fecralloy. Other catalysts which promote combination of CO and 2 to CO2 deposited on metallic substrates may be used.

Some advantages of a metallic substrate are that it yields a compact, efficient catalyst system which operates at a low temperature and thus consumes a minimum of power. Additionally, it has good resistance to thermal shock and mechanical failure unlike, for example, some prior art ceramic substrates and so has a long useful life particularly suited for the reheating and cooling involved each time the laser is switched on and off for use.

Factors which determine the quantity, disposition and operating temperature of the catalyst system include the volume, composition and pressure of the lasing medium, the pulse repetition frequency (in a pulsed laser) and the acceptable degradation in laser performance. In order to maintain equilibrium the catalyst system is arranged to cause recombination of CO and 02 at the same rate as dissociation of CO2.

In the past, sealed CO2 lasers have had limited useful lives due to the breakdown of CO2 and therefore a flowing gas laser having a system for renewing the lasing medium has been necessary where prolonged life has been required. The requirement for a renewal system has involved bulky peripheral apparatus and thus the laser has not been compact and easily portabie. However, by incorporating into a sealed laser a catalyst system comprising a catalyst deposited on a metallic substrate, wherein the catalyst promotes the oxidation of CO by 2 to CO2, a sealed, compact, portable laser with a long useful life is obtained.

Claims (6)

Claims

1. A CO2 laser having a lasing medium which includes CO2 wherein the lasing medium is in contact with a catalyst system comprising a catalyst mounted upon a metallic substrate, the catalyst being such as to promote combination of 02 and CO to CO2.

2. A CO2 laser as claimed in claim 1 wherein the laser is sealed.

3. A CO2 laser as claimed in claim 1 or claim 2 wherein the metallic substrate comprises aluminium-containing ferritic steel having an oxidised surface including alumina.

4. A CO2 laser as claimed in claim 3 wherein the metallic substrate additionally contains chromium and yttrium.

5. A CO2 laser as claimed in any previous claim wherein the catalyst is either platinum or palladium or both.

6. A CO2 laser substantially as herein described with reference to the accompanying drawings.